In 1650, suffering from frail health, Pascal retired temporarily from mathematics. However, in 1653, his health recovered and he wrote Traité du triangle arithmétique in which he described a convenient tabular presentation for binomial coefficients, the "arithmetical triangle", now called Pascal's triangle.

In 1654, prompted by a friend interested in gambling problems, he corresponded with Fermat on the subject, and of that collaboration was born the mathematical theory of probabilities. He later used a probabilistic argument, Pascal's Wager, to justify belief in God and a virtuous life.

In this new field, Pascal gained fame for his attack on casuistry, a popular ethical method used by Catholic thinkers in the early modern period, (especially the Jesuits). Pascal denounced casuistry as the mere use of complex reasoning to justify moral laxity. His writings on this subject, a defense of the JansenistAntoine Arnauld, appeared as the Lettres provinciales, or "Provincial Letters." This work incensed King Louis XIV of France who ordered in 1660 that the book be shredded and burnt.

Pascal's most influential theological work, the Pensées, was yet unfinished by his death, but a version of his notes for that book appeared in print in 1670, eight years after, and it soon became a classic of devotional literature.

Pascal died in Paris on August 19, 1662 and is buried there in the St. Étienne-du-Mont cemetery.